This invention relates generally to simplified feedthrough capacitor terminal pin subassemblies and related methods of construction, particularly of the type used in implantable medical devices such as cardiac pacemakers and the like, to decouple and shield undesirable electromagnetic interference (EMI) signals from the device. More specifically, this invention relates to a simplified and reduced cost ceramic feedthrough capacitor assembly and related installation method, including one or more filter capacitors which also form the hermetic seal. This eliminates the need for a separate and costly hermetic terminal subassembly which is common in the prior art. In the present invention, the ceramic feedthrough capacitor itself forms the hermetic seal and is adapted particularly for use in connecting a lead wire or electrode through a hermetically sealed housing to internal electronic components of a medical device while decoupling EMI against entry into the sealed housing. This invention is particularly designed for use in cardiac pacemakers (bradycardia devices), cardioverter defibrillators (tachycardia), neurostimulators, internal drug pumps, cochlear implants and other medical implant applications. This invention is also applicable to a wide range of other EMI filter applications, such as military or space electronic modules, where it is desirable to preclude the entry of EMI into a hermetically sealed housing containing sensitive electronic circuitry.
Feedthrough terminal pin assemblies are generally well known in the art for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. However, the feedthrough terminal pins are typically connected to one or more lead wires which effectively act as an antenna and thus tend to collect stray EMI signals for transmission into the interior of the medical device. In the prior art devices, the hermetic terminal pin subassembly has been combined in various ways with a ceramic feedthrough filter capacitor to decouple interference signals to the housing of the medical device. A primary feature of the simplified feedthrough terminal pin subassembly described herein is cost reduction which is accomplished by elimination of the separate hermetic terminal subassembly.
In a typical prior art unipolar construction (as described in U.S. Pat. No. 5,333,095), a round/discoidal (or rectangular) ceramic feedthrough filter capacitor is combined with a hermetic terminal pin assembly to suppress and decouple undesired interference or noise transmission along a terminal pin. The feedthrough capacitor is coaxial having two sets of electrode plates embedded in spaced relation within an insulative dielectric substrate or base, formed typically as a ceramic monolithic structure. One set of the electrode plates is electrically connected at an inner diameter cylindrical surface of the coaxial capacitor structure to the conductive terminal pin utilized to pass the desired electrical signal or signals. The other or second set of electrode plates is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing of the electronic device. The number and dielectric thickness spacing of the electrode plate sets varies in accordance with the capacitance value and the voltage rating of the coaxial capacitor. The outer feedthrough capacitor electrode plate sets (or "ground" plates) are coupled in parallel together by a metallized layer which is either fired, sputtered or plated onto the ceramic capacitor. This metallized band, in turn, is coupled to the ferrule by conductive adhesive, soldering, brazing, welding, or the like. The inner feedthrough capacitor electrode plate sets (or "active" plates) are coupled in parallel together by a metallized layer which is either glass frit fired or plated onto the ceramic capacitor. This metallized band, in turn, is mechanically and electrically coupled to the lead wire (s) by conductive adhesive or soldering, or the like. In operation, the coaxial capacitor permits passage of relatively low frequency electrical signals along the terminal pin, while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the conductive housing. Feedthrough capacitors of this general type are available in unipolar (one), bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (6) and additional lead configurations. The feedthrough capacitors (in both discoidal and rectangular configurations) of this general type are commonly employed in implantable cardiac pacemakers and defibrillators and the like, wherein the pacemaker housing is constructed from a biocompatible metal such as titanium alloy, which is electrically and mechanically coupled to the hermetic terminal pin assembly which is in turn electrically coupled to the coaxial feedthrough filter capacitor. As a result, the filter capacitor and terminal pin assembly prevents entrance of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
In the past, feedthrough filter capacitors for cardiac pacemakers and the like, have typically been constructed by preassembly of the coaxial capacitor onto or within a cylindrical or rectangular hermetically sealed terminal pin subassembly which includes the conductive pin and ferrule. More specifically, the terminal pin subassembly is prefabricated to include one or more conductive terminal pins supported within the conductive ferrule by means of a hermetically sealed insulator ring or bead. One type of hermetic terminal pin subassembly which is widely used in implantable medical devices employs an alumina ceramic insulator which, after complicated sputtering/metallization procedures, is gold brazed into a titanium ferrule. In addition, there are platinum lead wires which are also gold brazed to the alumina ceramic insulator to complete the hermetic seal. See for example, the subassemblies disclosed in U.S. Pat. Nos. 3,920,888; 4,152,540; 4,421,947; and 4,424,551. An improved design in the prior art which has substantially improved the volumetric efficiency is based upon surface mounting of a ceramic feedthrough capacitor planar array structure to one outer surface of a hermetic terminal with similar connection to the conductive pins (see the subassemblies disclosed in U.S. Pat. No. 5,333,095). In all of the prior art described above, the feedthrough capacitor is mounted to a separate and relatively costly hermetic terminal pin subassembly.
Although feedthrough filter capacitor assemblies of the type described above have performed in a generally satisfactory manner, the associated manufacturing and assembly costs are unacceptably high. For example, the manufacture of the separate hermetic terminal subassembly is very costly (in most instances the hermetic terminal pin subassembly costs more than the ceramic feedthrough capacitor). In addition, the subsequent installation of the ceramic feedthrough capacitor is time consuming and therefore costly. More particularly, as shown in FIG. 1 of U.S. Pat. No. 4,424,551, installation of the coaxial capacitor into the small annular space between the terminal pin and ferrule can be a difficult and complex multi-step procedure to ensure formation of reliable, high quality electrical connections. The method taught by the 4,424,551 patent teaches the injection of fluidic thermosetting conductive particles into first and second annular cavities (usually by centrifuge operations). This is, however, a time consuming and relatively expensive process.
Accordingly, there is a need for a novel feedthrough filter capacitor assembly that addresses the drawbacks noted above in connection with the prior art. In particular, a novel capacitor assembly is needed which eliminates the fabrication of a separate hermetic terminal pin subassembly and yet may be utilized in many of the same applications, where such subassemblies are now found. Additionally, the improved feedthrough filter capacitor assembly should lend itself to standard manufacturing processes such that cost reductions can be realized immediately. Of course, the new design must be capable of effectively filtering out undesirable electromagnetic interference (EMI) signals from the target device. The present invention fulfills these needs and provides other related advantages.